Smoothing of films under 300 micrometers in a calender stack is difficult, since the melt in the smoothing nip exhibits transition. Therefore it is practically impossible to equalize variations in thickness of the extruded web by a bead of thermoplastic compound on the roll intake in front of the nip. According to DE-AS 1 251 188, a satisfactory two-sided smoothing is indeed achieved if the nip is formed from a polished steel roll and a highly polished rubber roll. However, highly polished rubber rolls endure for only short running times. Also, pressure rolls made from other materials with a substantially lower E modulus than steel (for instance, polyamide--which, according to DE-C 3 013 576, can also be used) have a limited running time.
According to DE-C 3 421 800, a longer running time is achieved with a pressure roll which comprises a covering made from elastic material and, over the covering, a tubular thin-walled smoothing shell of polished steel. However, rolls of this configuration are more rigid than rubber rolls. This results in closely adjacent thickness variations in the melt web not being equalized, since the web does not contact the rolls with its complete surface. Optical defects are the result.